The invention relates to a measuring method in which a measuring signal, a part of which is reflected back, is transmitted to a transmission line and in which method the reflected signal is measured.
Signals between the equipment of a fixed transmission network in various locations are typically transmitted by means of signal conductors. A transmission path comprising several signal conductors is usually called a cable. Cables can be divided for instance by their installation sites into aerial, underground, channel, underwater and inside cables. In cables, a signal attenuates relatively quickly, which is why signals are amplified when necessary. Signal conductors are typically made of copper or an alloy, for instance.
Cables typically comprise several insulating layers and a sheath which protects the signal conductors from mechanical strain and humidity, for instance. The sheath can be of metal or a plastic or rubber compound, for instance. An individual signal conductor or an entire cable can, however, in some cases break or be damaged, in which case the transmission properties of the conductor deteriorate.
Various methods are used for checking the condition of a signal conductor. In a known method, a short voltage impulse is transmitted to the signal conductors forming a subscriber line, after which the return reflection of the transmitted signal is measured. This way, any discontinuities in the subscriber line can be found and the line repaired.
However, the above method provides the disadvantage that the implementation of the measuring apparatus executing the method is very complex. The measuring apparatus must detect a fast impulse, whose amplitude level can be very low, in the time domain. This requires great speed and sensitivity of the measuring apparatus. Since the measuring apparatus is of a complex design, its implementation is very expensive.
It is thus an object of the invention to implement a method and an apparatus implementing the method so as to solve the above problems. This is achieved by a method described in the preamble, which is characterized by transmitting a measuring signal to a transmission line being measured, measuring the magnitude of the measuring signal before it is transmitted to the transmission line, comparing the obtained measuring result with the magnitude of the measuring signal reflected back from the transmission line, calculating a reflection coefficient in the frequency domain by means of the obtained measuring results, and calculating an impulse response for the transmission line by means of the reflection coefficient in the frequency domain.
The invention also relates to a measuring apparatus which transmits a measuring signal to the transmission line, a part of which signal is reflected back to the measuring apparatus which comprises a processing means for measuring the reflected signal.
The measuring apparatus of the invention is characterized in that the processing means transmits a measuring signal to the transmission line, the processing means measures the magnitude of the signal before transmitting it, the processing means compares the measuring result with the magnitude of the reflected measuring signal, the processing means calculates a reflection coefficient in the frequency domain by means of the obtained measuring results, the processing means calculates an impulse response for the transmission line by means of the reflection coefficient in the frequency domain.
Preferred embodiments of the invention are set forth in the dependent claims.
The invention is based on the idea that a signal is transmitted to a transmission line and, on the basis of the signal reflected back from the transmission line, a reflection coefficient is calculated in the frequency domain, after which the frequency-domain signal is converted into the time domain.
The measuring method and apparatus of the invention provides several advantages. In the method, frequency-domain measurement is used, whereby a impulse response of the signal can be easily calculated. The measuring apparatus can be implemented with relatively easy connections, which makes it inexpensive to implement. The measurements performed by the measuring apparatus can be implemented with relatively slow components. In addition to this, the measuring apparatus can be implemented with low-sensitivity components.